1. Field of the Invention
The present invention pertains to apparatus and methods for detecting the presence or absence of material at a predetermined location. More particularly, the present invention relates to systems for determing the amount of material accummulated at a locale. Apparatus and methods of the present invention find particular application in the case of material storage in containers, such as bins, wherein the amount of material present may be determined by detecting the level of the material. Appropriate indicators, or warning devices, may be triggered ultimately by use of the present invention, and systems for adding material, or removing material, may also be operated in response to the detection of the material level by use of the present invention.
2. Description of the Prior Art
Several prior art systems available for measuring material level in bins include the use of mechanical probes which extend into the bin. Such a probe is made to vibrate, and the physical characteristics of the elastic disturbances, which are affected by the quantity of material present about the probe, are analyzed in terms of the quantity of material present.
Other prior art devices rely upon the variation of one or more electrical properties affected due to the presence of material at different levels within the bin. Thus, capacitive reactance of one or more probes may be affected to signal the amount of material present. The operation of an oscillator circuit, including a resonant probe, may be affected to operate a control system in response to the presence of material near the resonant probe.
U.S. Pat. Nos. 3,807,231 and 3,935,970 disclose a measuring system, and related control system, wherein the reactance of an antenna probe varies as a function of the level of material in the container adjacent the antenna. The antenna reactance controls the frequency of a transmitter signal, which is then combined with a constant frequency reference signal. The frequency of the resulting difference signal is used to operate a material level indicator and to provide control information for operating automatic systems for controlling the material level.
An amplitude-modulated detector circuit is disclosed in U.S. Pat. Application Ser. No. 844,040, filed Oct. 20, 1977 now Pat. No. 4,169,543. There, a crystal-controlled oscillator generates a constant-amplitude signal which is fed through a load resistor to an antenna probe circuit. Variations in the amplitude of this loaded signal, due to the presence of material to be detected about the antenna probe, are reflected in variations in the value of a d-c voltage level. This variable voltage level is combined with a reference voltage level and the resulting signal is fed to a switching circuit. The switching circuit provides an output signal depending on a comparison between the combined signal and a second reference voltage level.